Forbid attribute_filters={"key": None} case

[tcompa]

Second

attribute_filters={"key1": None, "key2": ["A", "B"]}
-> require that image has attribute key2 equal to A or B, and do not apply any filtering for key1

This is due to:

• Remaining backwards-compatible (since up to v2.10 setting an attribute value to None would disable that filter).
• The fact that we often had to merge different sets of filters (not any more, see Replace attribute filters rather than merging them #2155) and then this was way to represent a patch that would disable a previously set filter.

The latter reason does not hold any more, and I guess the first one is not very relevant in real-life usage. Thus I'd be in favor of deprecating this option. Either way, we'll have to review this (because it's currently not fully backwards compatible, in the API schemas).
Originally posted by @tcompa in #2182

[tcompa]

@tcompa Agreed to all! Looks good to me. I'll review additional type filter questions on that issue separately

attribute_filters={"key1": None, "key2": ["A", "B"]}
...
Thus I'd be in favor of deprecating this option.

For job attribute filters, we can remove this without an issue. Given that the only source for defaults for job attribute filters are the dataset attribute filters, I don't see a use for keeping the support for None = unset the filter anymore as well.
Agreed on deprecating it.

Originally posted by @jluethi in #2182

[tcompa]

• Update our custom type for attribute filters
• Double check whether we need to explicitly forbid this case in some fractal_server.image or API module
• Update data-migration script, so that any occurrence of this "key": None pair is removed, with a warning.

[tcompa]

Closed with #2202